30 TAU (30 Tauri). The main figure of Taurus (the Bull) is so dominating, the Hyades (which make the Bull's head) so prominent, the Pleiades so sparkling, that we pay little attention to the extended forward (westerly) part of the constellation as it butts up against Aries and Cetus. Taurus's head points like an arrow at third magnitude Lambda Tau and, continuing the line further, right at fifth magnitude (5.10) 30 Tauri, which (carrying neither proper name nor Greek letter) is best known by its Flamsteed number. The star is well-known too for its relative youth. 30 Tau, at a respectable distance of 565 light years, is a visual binary that consists of a luminous hot class B (B3) dwarf coupled with a much fainter ninth magnitude (9.41) class F (F5) star that bears some resemblance to the Sun. Their on-the-sky separation of just under 10 seconds of arc leads to an orbit with a radius of at least 1650 Astronomical Units. The temperature of the bright star is problematic, the measured value of 16,400 Kelvin rather too cool for a B3 star (which should be more like 18,000). The correction to brightness for interstellar dust is rather uncertain as well, ranging between one and three tenths of a magnitude. All values, however, point to the brighter component as a grand star with a luminosity between 900 and 1300 times that of the Sun, a radius between 3.5 and 4 solar, and a mass between 5.3 and 6 solar. (Even the rotation speed is argued, values falling between 20 and 50 kilometers per second, the slowest giving a rotation period of under 10 days, as opposed to the Sun's 25. A lot of work clearly remains.) 30 Tau is a "Landroos star," after a catalogue of similar stars by K. P Landroos that features very young binaries consisting of luminous class O and B stars coupled with much lower luminosity class F, G, and K dwarfs (or "pre-dwarfs"). The class F companion in 30 Tauri (30 Tau B), with a temperature of around 6600 Kelvin (the Sun itself at 5780), radiates at a rate of 4 or 5 times that of the Sun, which leads to a radius of 1.7 solar and a mass around 1.3 solar. The age of the system has been estimated at under 28 million years. While the luminous class B star (30 Tau A) is a full-blown dwarf, 30 Tau B may be still settling down in its birth process to become a true "zero-age" main sequence dwarf, from which point it will begin to evolve rather like the Sun. The two take at least 25,000 years to orbit. From 30 Tau B, 30 Tau A would appear as a magnificent star shining with the light of 100 full Moons. While X-ray emission suggests the faint star is magnetically active, there is no evidence for a planet-forming dusty disk (like there is around many youthful stars), any disk perhaps disrupted by radiation from 30 Tau A.
Written by Jim Kaler. Return to STARS.